During wet processing of semiconductor wafers, the wafers are exposed to various reagents. Examples of such process steps include etching, photoresist stripping, and prediffusion cleaning. After cleaning, the semiconductors are rinsed and dried. In order to process wafers efficiently, they must be suspended in the various reagents with some type of holder or carrier.
End effectors are also referred to in the art as boats, carriers, rigs, cassettes, etc.
Semiconductor wafer carriers are used in several different types of processing. For example, there is high temperature diffusion processing that is carried out at temperatures from 600.degree. to 1200.degree. C. There is also wet processing which is carried out at temperatures less than 180.degree. C. The high temperatures of diffusion processing require that a wafer carrier designed for use in diffusion processing must provide for substantial expansion and warping of the wafer. In wet processing, expansion is not a concern. Other concerns that mandate carrier design include; (1) even and adequate contact of the wafer surfaces with the reagents, and (2) the avoidance of trapping gases under the rails of the end effector as the end effector is lowered into the liquid. This gas can be released later and enter a trench on the semiconductor wafer and block cleaning or etching of the area under the bubble.
Manual wet processing treatment typically makes use of a regular wafer boat or wafer cassette to which a boat handle is attached for introducing and removing the boat from the process chemistry. The standard wafer boat for eight inch wafers has a groove larger than the width of the wafer thickness. The wafers move back and forth within their slots during wet processing. The standard boat configuration for an eight inch low profile cassette cups the entire lower half of the wafer with a six mm intrusion onto the wafer surface. The standard configuration for a high profile cassette contacts 3/4 inch of the wafer diameter. Typically, boats are made of a fluoropolymer material, peek, or quartz.
In automated wet processing, the typical boat, transport rack or cassetteless processing grips the wafer with an end effector using a standard six mm deep v shaped groove that cups the wafers or holds them with a minimum of 4 points of contact.
Wherever the surface of a wafer comes into contact with the wafer carrier, there exists the possibility for imperfections in the surface of the wafer which lowers overall yield. Most of the above carriers and other prior wafer carriers fall into one of three categories, namely, "four member" carriers, "three member" carriers and "shell boats". All of these designs have a great deal of wafer surface to carrier contact. For instance, a four member carrier usually has four points of contact that intrude onto the wafer surfaces. A shell boat has contact with a wafer around the entire periphery of the wafer.
U.S. Pat. Nos. 4,493,418, 4,687,097, 4,471,716, 4,724,963, 4,949,848, and 5,111,936 are characteristic of wafer processing cassettes known in the art for the wet processing of wafers. In each of these patents, the wafers are held in place by cassettes that have a great deal of contact with the wafer surfaces. In addition to the large regions of intrusion onto the wafer surfaces, these cassettes hold the wafers vertical with no disclosed leaning angle.
Some of the wafer processing carriers or cassettes known in the art of high temperature diffusion disclose cassettes that hold wafers at a leaning angle from vertical. However, these cassettes or carriers do not tightly hold the wafers with minimal intrusion onto the wafer surfaces. Moreover, this design has not been employed in carriers designed for wet processing.
U.S. Pat. No. 4,981,222 ("the '222 patent") discloses a wafer supporting method and apparatus having at least two wafer supporting slots for each edge-wise oriented wafer in a plurality of coaxial edge-wise aligned wafers as depicted in FIG. 1 of the '222 patent. The wafers are supported by either 3 or 4 round rods as depicted in FIGS. 3, 5 and 6 of the '222 patent as well as column 4, lines 34-35 and column 6, lines 14-18. The wafer carriers disclosed are made of quartz and are designed for diffusion processing at high temperatures and allow for edge to edge and front to back translational movement during processing and handling.
Each slot of the '222 patent has a pair of inwardly directed bevels at the upper entrance level that may be at an angle of about 45.degree. to 60.degree. from the slot's geometrically-centered axis. The bevels lead downwardly into a pair of parallel slot walls that are spaced apart by about twice the thickness --2t-- of the wafer. The rightmost wall is tangent to an arcuate-shaped bottom wall circle having a radius of --2t--, where t is the thickness of a wafer to be held in the slot. The longer wall of the parallel slot walls is the wall against which the wafer will lean. Gravity will cause the wafer to drop to the lowest point at the rounded bottom of the slot. Accordingly, the '222 patent discloses a wafer cassette having slots which are twice the width of the thickness of the wafer and a rounded bottom wall thereby holding the wafer loosely in place by the force of gravity.
U.S. Pat. No. 4,515,104 discloses a quartz boat for diffusion processing formed with a plurality of spaced parallel rods having slots for supporting a series of semiconductor wafers in a generally edgewise, spaced, parallel position. The slots continue to the ends of the rods, and the cross member supporting the rods are spaced inwardly from the rod ends, such that a series of boats may be arranged with the rods in end-to-end relation and the wafer space between adjacent boats is the same as the wafer space in the middle of a boat. Little detail is disclosed regarding the shape of the slots or the freedom of movement the wafers have when seated in the slots. The drawings disclose a boat with 4 points of contact intruding onto the wafer surfaces.
U.S. Pat. No. 4,053,294 discloses a semiconductor boat or carrier with three slotted points of contact intruding onto the wafer surfaces. The grooves are beveled and symmetrical about the centerline of the groove.
A problem associated with most of the above-described designs is that die yield at the edges of the wafers is sacrificed because the active surface area of each wafer is diminished by the intrusion of the end effector. The further the end effector intrudes into the wafer surface, the larger the area on the outer surface of the wafer that is rendered useless. Particle generation can also be a problem where the wafers are free to move around within the grooves. The wafers rattle about within the grooves, colliding with the inner surfaces of the groove thereby chipping away at the wafer or the rack which produces particulate contamination.
Another problem with boats such as those above-described is that they are difficult to clean. The greater the number of grooves, the more difficult the carrier is to clean. Also, cassettes with deep grooves are more difficult to clean. Horizontal surfaces and points where rods connect result in places that trap liquids and particles.
Another problem with many of the above-mentioned wafer carriers is that they are made of materials which have undesirable properties for wet processing. For instance, teflon absorbs process chemicals which are later outgassed, forming a haze or unwanted etching on the surfaces of the adjacent wafers as well as damage to other equipment downstream of the wet station. Teflon also creates particulate contamination thereby decreasing wafer yield. Teflon is soft and is easily scored by silicon wafers and this can create particle contamination as well as trap the edge of the wafer within the teflon. This can lead to wafer breakage when the wafer is moved perpendicular to the direction it is held or trapped (which frequently occurs in a spin dryer). Carriers made from PEEK (poly ethyl ether ketone) are not suitable for processes completed at or above 100.degree. C. such as piranha or phosphoric cleaning. Quartz carriers are too fragile and the grooves deteriorate. This often leads to adherence of the wafer to the quartz. Removal of a disc that has adhered to the quartz carrier usually results in breakage of the wafer. Also, quartz carriers are incompatible with hydrofluoric acid.
Accordingly, it is an object of this invention to provide for an end effector that holds a plurality of wafers tightly with only two points of contact that minimally intrude onto the wafer surfaces.
It is further an object of the present invention to design a wafer end effector that is easily cleaned.
It is further an object of the present invention to design a wafer end effector that holds a plurality of wafers at a slight angle from vertical.
It is yet another object of the present invention to provide a lightweight, sturdy end effector made of a material which minimizes particle generation and is chemically inert.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.